1. Field of the Invention
The present invention relates to anodes for lithium-ion (Li+) rechargeable batteries and lithium-ion rechargeable batteries including the anodes.
2. Description of the Related Art
A lithium-ion rechargeable battery is a kind of non-aqueous electrolyte secondary cells in which lithium ions move between an anode (negative electrode) and a cathode (positive electrode) during charging and discharging processes. Lithium-ion rechargeable batteries have higher energy density than other secondary cells such as nickel hydride rechargeable batteries and are widely used as secondary cells for supplying power to mobile electronic devices.
Meanwhile, as mobile electronic devices have become more advanced and more compact in recent years, there has been growing demand for smaller and higher-capacity lithium-ion rechargeable batteries to be used as power supplies for such devices. In order to meet this demand, it is indispensable to achieve higher capacity of negative-electrode active materials.
Carbon-based materials have conventionally been used for negative-electrode active materials. They absorb/desorb lithium ions by intercalating/deintercalating lithium-ions between graphene layers. The theoretical specific capacity of carbon-based materials is 372 Ah/kg. Negative-electrode active materials composed of carbon-based materials already have actual capacities close to the theoretical specific capacity, and there is little room to dramatically improve their capacities.
Currently, therefore, a quest for alternative materials to carbon-based materials is being pursued vigorously, and there is a growing interest in alloy negative electrodes (or metal negative electrodes) that promise to have high capacities. Alloy negative electrodes (or metal negative electrodes) perform charge/discharge reactions through alloying/dealloying reactions, which are indicated in the formula: xLi++M+xe− LixM, wherein M is a metal element. For example, the theoretical specific capacity of tin (Sn) and that of silicon (Si) are 990 Ah/kg and 4200 Ah/kg, respectively, which are several to 10 or more times higher than the theoretical specific capacity of carbon-based materials.
On the other hand, the volumes of these materials change largely upon charging and discharging the battery. It is known that tin expands/contracts by 360% and silicon expands/contracts by as much as 420% during the intercalation and deintercalation of lithium ions. The structure of an electrode cannot be maintained due to stress caused by this large volume change accompanied by charging and discharging the battery. Thereby, the cycle characteristics of these materials are unfortunately inferior compared to those of carbon-based materials to a remarkable extent. In other words, alternative materials must be considered in terms of improving cycle characteristics.
For example, a negative-electrode active material for lithium secondary cells has been suggested in JP-A 2009-32644, the negative-electrode active material having a number of Si cores and an alloy matrix surrounding the Si cores. The alloy that composes the alloy matrix contains at least one element x selected from Al (aluminum), Sn, Ag (silver), Bi (bismuth), and Zn (zinc) and at least one element y that is different from the element x and selected from Co (cobalt), Ni (nickel), Ag, Al, Fe (iron), Zr (zirconium), Cr (chromium), Cu (copper), P (phosphorus), V (vanadium), Mn (manganese), Nb (niobium), Mo (molybdenum), In (indium), and rare earth elements. The alloy that composes the alloy matrix preferably contains at least one element z that is different from the element x and the element y and selected from Ag, Al, Bi, P, Sn, Ti, and Zn. According to JP-A 2009-32644, there can be provided a negative-electrode active material for lithium secondary cells, the negative-electrode active material being capable of improving cycle characteristics of lithium secondary cells and having excellent productivity.
Also, a battery provided with a positive electrode, a negative electrode, and an electrolyte has been suggested in JP-A 2004-22512, the negative electrode including a porous body composed of an elemental, alloy, or compound metal or metalloid element capable of alloying with lithium. The porous body is a continuous solid substance containing holes. According to JP-A 2004-22512, the negative-electrode material of JP-A 2004-22512 has not only a high capacity but also excellent charge-discharge characteristics, since its porous body makes it less likely for the structure to collapse by absorbing the volume change upon absorbing and desorbing lithium.
In addition, a negative electrode active material for lithium secondary cells has been suggested in JP-A 2004-214054, the negative electrode active material being composed of an aggregate of Si porous particles formed with a number of voids therein. The voids have an average hole diameter of between 10 nm and 10 μm, and the aggregate has an average particle diameter of between 1 μm and 100 μm. According to JP-A 2004-214054, there can be provided a negative-electrode active material that can prevent: pulverization due to expansion and contraction of the volume of the active material upon charging and discharging the battery; peeling of the active material from the current collector; and loss of contact of the active material with the conductive material.
However, with the negative-electrode active material for lithium secondary cells described in JP-A 2009-32644, cycle characteristics are not sufficiently improved. The negative-electrode material described in JP-A 2004-22512, which is a porous body, may pulverize due to lack of sufficient mechanical strength. Since the negative-electrode active material for lithium secondary cells described in JP-A 2004-214054 is composed of an aggregate of silicon porous particles and has no other matrix that contributes to stress relaxation, relaxation of stress caused by charging and discharging the battery is likely to be insufficient. Also, composed only of silicon, it unfortunately has poor electrical conductivity.